1. Technical Field
The invention relates to a method for coating an optical fiber.
2. Description of the Prior Art
Optical fibers, in particular glass fibers, are conventionally provided with a surface coating to protect the fibers from mechanical damage and contamination, to improve handling and, if necessary, to differentiate the fibers by color. In a method described in EP 509 487 A2, the fiber is initially drawn from a preform in a furnace in a manner known in the art. After the fiber has sufficiently cooled down, the fiber passes in the longitudinal direction through a coating unit where a liquid hardenable coating material is continuously applied to the fiber surface. The viscosity of the coating material at room temperature (25.degree. C.) is between 2000 and 6000 mPa.multidot.s. Commercially available coating materials have a viscosity of approximately 2500 to 5000 mPa.multidot.s and are typically processed at temperatures between 25 and 45.degree. C. for easy handling.
In a coating unit which is illustrated in EP 619 275 A2, the coating material is pressurized inside a volume into which the fiber enters through a guide nozzle. The fiber exits from the coating material through a coating nozzle, wherein the walls of the coating nozzle are spaced apart from the surface of the fiber. The fiber pulls the coating material which adheres to the fiber due to surface adhesion, from the coating unit through the coating nozzle, so that the fiber becomes coated with a liquid film. The material is hardened in a subsequent hardening unit, preferably by ultraviolet radiation. Plastic materials, in particular UV-hardenable resins with polymers containing urethane groups, are commonly used.
With the process just described, coatings having two or more layers are applied to glass fibers in mostly identical consecutive coating and hardening units. A relatively soft primary coating and a harder secondary coating for mechanical protection are preferred. To distinguish the different fibers by their color, the secondary coating may include pigments. Alternatively, an outer color coating may be applied in an additional process step. UV-hardenable materials are increasingly also used to color the fibers since inks that contain solvents are hazardous for the environmental and require a comparatively long transport path to dry if the transport speed of the fiber is high.
The quality of the applied coatings is often inadequate particularly at fiber velocities in the range above 1000 m/min. In particular, the coating material is frequently distributed asymmetrically relative to the fiber axis, i.e. the fiber is located eccentrically in the surrounding material. The same problem exists also with UV-hardenable inks, because the coating film requires a minimum thickness of at least several micrometers. Moreover, the liquid coating film frequently tears away from the surface of the fiber at velocities above about 1000 m/min, rendering the fiber useless.